How to apply scale to multi user?
I know you said no single user but your goal is to keep them linked so here's
my workaround:


unlink only one in object data panel 
apply the scale to this one
link the others to it in the object data panel


It can be a little bit of work in relation to the number of linked objects but it works. I had 25 objects sharing the same data and it took only a few seconds to finish.